Wireless communication systems are widely deployed to provide various types of communication content such as voice, video, data and so on. These systems may be multiple-access systems capable of supporting simultaneous communication of multiple mobile devices with one or more base stations.
Reductions in both the size and cost of mobile devices and base stations may increase their marketability. Devices in wireless communication systems are increasingly configured to operate on multiple radio frequency (RF) bands utilizing multiple transmission technologies. However, a device that operates on multiple radio frequency (RF) bands has traditionally required a separate duplexer for each radio frequency (RF) band. These separate duplexers have design constraints that prevent them from being used in integrated circuits. Instead, discrete duplexers have been used, increasing both the cost and printed circuit board area.
One basic characteristic of a frequency division duplex (FDD) system is that the downlink and uplink operate simultaneously at different frequencies. For proper operation, isolation between the transmitter and the receiver is required. Duplexers typically provide this isolation but a hybrid transformer may also provide the isolation. In order for isolation to be obtained using a hybrid transformer, the termination load must match (or be a scaled replica of) the antenna impedance. This applies to all frequencies where isolation is required. Benefits may be obtained by using hybrid transformers in systems where the uplink and downlink operate in the same frequency.